Removal of bullnose trim/ outward 90 degree corner trim insert from plank walls

[Stewfish]

On the corners of my 85 20' Minnie Winnie winnebago booth and walls that face outward, (convex corners) there is a 3/4 round bullnose trim. It slides down into a bracket that screws the corners together. I don't see a way to take if off the walls. I was able to slide the corner round off the cabinet face frame to remove the frame and replace the mdf shelves with sanded plywood then put the frame, bracket, and then trim back. Also the booth trim slid up and out and I replaced it since the bottoms we're cracked. I removed a small closet and used the corner trim for the booth. It seems hard and not flexible so how do I remove the trim without cracking it? Didnthey add it before putting on the roof? I would like to take the wall off the outside of the toilet/shower combo so I can make it easier to replace the cracked shower pan, extend the new pex pipe to the toilet, add white plastic FRP panels to the walls, and put in the new toilet.

Searching only brought up how to replace the outside corner trim

Hope this all makes sense, thanks in advance!